The present invention relates to techniques effective upon application to a high frequency power amplifier circuit used in a radio communication system such as a cellular phone, and so forth, for amplifying and outputting high frequency signals, and the radio communication system using the same, and more particularly, to techniques for improving detection accuracy of output power of a radio communication system having a detection circuit for executing detection of output power, necessary for feedback control of a high frequency power amplifier circuit, by use of a current detection method.
Generally, a high frequency power amplifier circuit for amplifying signals after modulated is installed in an output unit at the transmitting end of a radio communication device (mobile radio communication device) such as a cellular phone, and so forth. A conventional radio communication device is provided with an automatic power control circuit (APC circuit) for generating an output control signal Vapc for effecting feedback control of a high frequency power amplifier circuit by detecting an output level of the high frequency power amplifier circuit to compare a transmit demand level (an output level designation signal) with a detection signal in order to control an amplification factor of the high frequency power amplifier circuit such that output power corresponds to the transmit demand level delivered from a base band circuit or a control circuit of a microprocessor and so forth (refer to, for example, Patent Document 1). Detection of an output level, necessary for the feedback control, has generally been executed by use of a coupler in the past.
A detection method of the output level of the conventional high frequency power amplifier circuit, using the coupler, renders it difficult to downsize a module. Further, in the case of using the coupler, there are times when a reference voltage is impressed on one end of the coupler in order to enhance detection sensitivity. In such a case, a drawback results in that a burden on a set maker will increase because of the needs for optimum setting of the reference voltage, and adjustment in voltage and so forth, according to variations in components. The use of the coupler also results in a drawback in that a relatively large power loss will occur.
Accordingly, the applicant for the present invention have developed the invention relating to a radio communication system based on a current detection method, comprising a current detection transistor for receiving an input signal of a power amplification transistor for amplifying a high frequency signal, and outputting a current proportional to a current flowing in the power amplification transistor, and a current mirror circuit for transferring the current of the current detection transistor, wherein a current generated by the current mirror circuit is converted into a voltage to serve as an output level detection signal, thereby controlling an output level by comparing the output level as detected with a transmit demand level, and have since submitted an application for the patent (Patent Document 2).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-151310
[Patent Document 2] Japanese Patent Application No. 2000-523757